Aqueous corrosion of metals is an electrochemical process involving anodic oxidation of a metal in a solution and cathodic reduction of species from the solution. This process is often monitored using an electrochemical technique, such as Linear Polarization Resistance (LPR), Tafel extrapolation and Electrochemical Impedance Spectroscopy (EIS). However, all of these techniques suffer from one or more significant drawbacks. For example, the LPR and EIS techniques can be used for instantaneous corrosion rate measurements only if the anodic and cathodic Tafel parameters (b.sub.a and b.sub.c, respectively) are known. The Tafel extrapolation technique permit determination of the corrosion rate and the Tafel parameters but is not suitable for instantaneous corrosion rate measurements because the system must be polarized over a wide potential range such that the measurement is time consuming and the surface of the metal is affected by the measurement.
Other electrochemical techniques rely on the fact that the corrosion process is non-linear, and that applying one or more sinusoidal signals will generate a response current at more frequencies than the frequencies of the applied signal. Thus, the corrosion rate can be determined by measuring a response current to the sinusoidal signals. One of these techniques, known as the Faraday rectification technique, involves measuring the response current at a "zero" frequency, that is measuring a direct current (DC). The Faraday rectification technique can be used if at least one of the Tafel parameters is known. Another technique, known as harmonic analysis, enables the corrosion rate and both Tafel parameters to be obtained with one measurement by analyzing the harmonic frequencies. Harmonic analysis has been used for corrosion rate measurements in acid media with and without inhibitors. A special application of harmonic analysis is Harmonic Impedance Spectroscopy (HIS), where the harmonic current components are transformed to harmonic impedances. HIS has been used to measure corrosion rates of polarized systems.
In corrosion research, virtually no attention has been given to intermodulation techniques. With the intermodulation technique one or more sinusoidal signals of different frequencies are applied to a corroding system and response currents measured. The alternating current (AC) responses include response currents at harmonics or multiples of the frequencies of the applied signals (.omega..sub.1, 2.omega..sub.1, 3.omega..sub.1, . . . , .omega..sub.2, 2.omega..sub.2, 3.omega..sub.2, . . . ), and response currents at the intermodulation frequencies (.omega..sub.1.+-..omega..sub.2, 2.omega..sub.1.+-..omega..sub.2, 2.omega..sub.2.+-..omega..sub.1 . . . ). With the intermodulation technique, just as with harmonic analysis, it is possible to determine a corrosion rate without prior knowledge of the Tafel parameters. The intermodulation technique as such has been used satisfactorily in semi-conductor research. However, until now the intermodulation technique has never been successfully used for monitoring corrosion rates.
A further problem with all of the above techniques is that they do not provide a way to validate the measured response currents. Small currents have to be measured to determine a corrosion rate. These currents are easily influenced by background noise or any other kind of (electrical) disturbance. Using improperly measured data for the calculation of a corrosion rate can result in misleading results. In case of under estimation of corrosion rate, without a mechanism for validating the measured data an error in determining the corrosion rate is often not discovered until considerable damage has been done.
Accordingly, there is a need for a method and apparatus for quickly and continuously monitoring a corrosion rate of a corroding system without prior knowledge of Tafel parameters of the system. There is also a need for a method and apparatus for determining the Tafel parameters of the system. There is a further need for a mechanism for validating the measured data used to calculate the corrosion rate.